Coal tar, and especially the high temperature coal tar recovered as a by-product of metallurgical coke manufacture, can be converted by distillation to a pitch that has utility as a binder component in the production of anodes used in aluminum reduction cells and graphite electrodes used in electric-arc furnaces. With controlled quality of the binder pitch, it is possible to achieve advantageous properties in the anodes, such as high mechanical strength, good electrical conductivity, and low carbon consumption rates during the electrolysis process. However, certain impurities in the tars, which are transferred to the product pitch, may exert deleterious effects. These impurities are generally quantified by a solvent extraction technique employing quinoline as the solvent. The quinoline insolubles (QI), which denote the degree of contamination of the tar, consist essentially of coal-derived solids (coal, coke, cenospheres) and by-product-derived solids (carbon blacks, pyrolysis blacks). Coal-derived contaminants, in addition, contain the inherent mineral matter associated with the feed coal to the coke ovens, and various of the elements in the mineral matter (Na, Si, V, P) are in themselves undesirable as components in the aluminum reduction cells. It is, therefore, of critical importance to be able to remove a large proportion of the solid particulates in the coal tar and thus render the tar suitable for production of anode-binder pitch, as well as other related products.
The most common techniques applicable for upgrading tar quality include filtration, gravity settling, and centrifugation. Because of the "sticky" nature of coal tar, filtration is not easily accomplished, as the tar solids readily blind the filter media and produce unacceptably low filtration rates even when large quantities of filter aids are employed. Depending on the viscosity of the tar, simple gravity settling may only be partially effective, and the yield of usable tar may therefore be low.
Depending on the extent of contamination of the tar and the resultant viscosity, centrifugation may effect a moderate-to-high degree of purification. A serious shortcoming of centrifugation, however, lies in the co-production of a thickened bottoms (sludge) fraction that is not amenable to ready disposal. Proper disposal of this sludge often requires transporting the material to expensive landfills. Because of the tarry nature of the material, it presents serious handling problems.
In U.S. Pat. No. 4,036,603, incorporated herein by reference, coal tar is centrifuged to produce a liquid phase consisting of tar substantially free of solids and a solid phase consisting of solid matter wetted with tar. To overcome the disposal problem for the solid phase, this solids phase is combined with solid carbon-containing material, such as coal or coke dust, and mixed in a screw mixer to improve the handling properties. This solid material can then be readily transported for use or disposal. One of the problems with this process is that considerable valuable chemicals in the liquid tar are lost with the solid matter, or at least not readily recovered without complete reprocessing through coke ovens or the like. An additional problem is that the liquid phase often contains such a high solids content that the pitch derived from the process cannot even be utilized for a binder for electrodes. For a useful binder pitch, the QI level should be between about 10 and about 20 weight percent, and ash content should be below about 0.30 weight percent.
Some of the prior art, such as U.S. Pat. No. 4,264,453, incorporated herein by reference, also requires special non-aromatic solvents which results in extremely high costs for the process and results in contamination of the various coal-tar products (pitch and distillates) which must, by specification, be wholly aromatic.